Adding coupon process after the fact

Is there a way to add a coupon to a customer payment of a current subscriber? We have a few subscribers that were beta testers in our software subscription that I would like to reward. I thought I would give them $5.00 off for a few month and thought I could just add a coupon to their current subscription. I don't see a way of doing that.

Another question. Looks like the Larry's coupon Plugin allows you to give a coupon to an existing member. What I am looking for, and this may still do it, is the ability in the admin control panal to add a coupon to a current subscription. Either edit the payment and add coupon or add a payment with the coupon already applied. I don't want the user to need to log in to apply the coupon, I want to manage it myself. Does the Larry's coupon manager allow that?